The present invention generally relates to optical focusing. More particularly, the invention provides an apparatus and method for adjusting an objective lens and/or a sample stage. Merely by way of example, the invention has been applied to a scanning electron microscopy system. But it would be recognized that the invention has a much broader range of applicability.
Controlling critical process parameters may typically include assessing the performance characteristics of semiconductor fabrication processes such as resolution capability, across chip linewidth variations, and across wafer linewidth variations. As the dimensions of semiconductor devices continue to shrink with advances in semiconductor materials and processes, however, the ability to examine microscopic features and detect microscopic defects in semiconductor devices has become increasingly difficult.
During each semiconductor device fabrication process, defects such as particulate contamination and pattern defects may be introduced into the semiconductor devices. Such defects may be isolated to a single semiconductor device on a semiconductor wafer containing several hundreds semiconductor devices. Alternatively, the defects may be repeated in each semiconductor device formed across an entire semiconductor wafer. Isolated defects may be caused by random events such as an unexpected increase in particulate contamination in a manufacturing environment or an unexpected increase in contamination in process chemicals used in fabrication of the semiconductor devices. Repeated defects may be systematically caused by contamination or defects on a reticle.
Defects on semiconductor wafers may typically be monitored manually by visual inspection. Defects that may be visible to the human eye are limited by a lateral dimension 100 μm.
Automated inspection systems were developed to decrease the time required to inspect a wafer surface. Such inspection systems may typically include two major components such as an illumination system and a detection system. An illumination system may include a light source such as laser that may produce a beam of light and an apparatus for focusing and scanning the beam of light. Defects present on the surface may scatter the incident light. A detection system may detect the scattered light and may convert the detected light into electrical signals that may be measured, counted, and displayed on an oscilloscope or other monitor.
Systems used to manufacture semiconductor devices such as processing tools, metrology tools, and inspection tools may include a height sensor. A height sensor may be used to position a wafer within a system prior to the processing of the wafer. Height sensors may also be used in different configurations for different applications. Several height sensors that may be used as focusing sub-systems for processing, metrology, and inspection systems are currently available. But these height sensors often do not have sufficient effectiveness for automatic focusing in certain scanning electron microscopy systems.
Hence it is highly desirable to improve techniques for automatic focusing.